Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
Pitch systems are employed for adapting the position of a wind turbine blade to adapt to varying wind conditions. In this respect, it is known to rotate a wind turbine blade in such a way that it generates less lift (and drag) when the wind speed increases. In this way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blade towards its vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance.
Pitch systems generally comprise an electric or hydraulic motor which, through the use of reduction gearing (sometimes referred to as a “reductor”, or as a “reduction drive”), drives an actuating gear. Said actuating gear meshes with an annular gear provided on the wind turbine blade to set the wind turbine blade into rotation. Other actuating mechanisms operated by a pitch motor are however also known.
It is further known to provide an individual pitch system (comprising a separate motor and separate control) for each wind turbine blade of a rotor. It is also known to provide a common pitch system wherein the pitch angle of the blades is the same for all blades on a rotor. Such a common pitch system may comprise a single motor or may comprise a plurality of motors, one for each blade.
A control strategy of a pitch system that is often employed in variable speed wind turbines is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed (for example approximately 4 m/s-15 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine. Above the nominal speed (for example from approximately 15 m/s-25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. When the wind turbine is not operating, the blades may assume a vane position (e.g. at or around 90° pitch angle) to minimize the loads on the blades. During most of the wind turbine's life, a blade may however be in the below rated pitch position. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design.
In pitch systems comprising an annular gear (or “ring gear”), the annular gear is generally provided along an inner perimeter or outer perimeter of the pitch bearing. The pitch bearing comprises an inner ring and an outer ring and one or more rows of rolling elements between them. The inner ring may be connected with the hub and the outer ring may be connected with the blade or vice versa. The annular gear is provided on either the inner or outer bearing ring and is generally machined as an integral piece with the bearing.
Since the loads transmitted by the pitch motor and gearing are heavy, alternating in direction, cyclical and mainly concentrated on a reduced sector of the annular gear, the annular gear can be subjected to wear, especially in the flanks of the teeth. For example, repetitive contact between teeth of the annular gear and a pinion of the pitch system may cause e.g. fretting. Since the below rated pitch position is the prevailing position for most wind turbines, the contact between the teeth and its consequences is concentrated on the same teeth. The annular gear may thus require regular maintenance, which may be complicated and expensive.
One solution that has been proposed for reducing the wear of the annular gear is an induction hardening of the teeth of the gear. This solution however is rather expensive.
The present invention has the objective to provide a wind turbine rotor that at least partially solves one or more of the aforementioned problems.